The present invention relates to semiconductor materials required for high frequency devices, and in particular, relates to high-resistivity silicon carbide materials.
The present invention relates to modern semiconductor electronic devices and the semiconductor materials required for those devices. Semiconductor materials are useful based on their intrinsic properties as well as the properties they exhibit when doped with donor or acceptor atoms. Additionally, other semiconductor devices require portions that are insulating (of which the most familiar are the oxide insulators formed when silicon is oxidized) or semi-insulating. In particular, semi-insulating (sometimes referred to as xe2x80x9chigh resistivityxe2x80x9d) materials are useful as the substrates for a number of devices such as (but not limited to) field-effect transistors (xe2x80x9cFETsxe2x80x9d), an exemplary one of which is the metal-semiconductor field-effect transistor, commonly referred to as the xe2x80x9cMESFET.xe2x80x9d Although the terms xe2x80x9csemi-insulatingxe2x80x9d and xe2x80x9chigh-resistivityxe2x80x9d are often used interchangeably, the invention discussed herein is best expressed in terms of high resistivity, and this convention will be used consistently throughout the Background and Description, with the understanding that such use is helpfully descriptive, rather than functionally limiting, of the invention described and claimed herein.
The theory and operation and structure of various types of FETs are generally well known to ordinary skill in this art, and thus, will not be described in detail herein. In a simple model, however, a MESFET is formed on a high-resistivity or semi-insulating substrate by placing an epitaxial layer of conductive p or n-doped material on the substrate. Source, gate, and drain contacts are then made to the epitaxial layer, and when a potential (voltage) is applied to the gate, it creates a depletion region that pinches off the channel between the source and drain thereby turning the device off.
Accordingly, the performance of the MESFET depends on the quality and characteristics of the semiconductor epitaxial layer, and upon the quality of and characteristics of the high-resistivity substrate.
As wide-band gap materials, such as silicon carbide (SiC) and the Group III nitrides (e.g. GaN, AlGaN and InGaN) have become more commercially feasible, the potential for producing higher frequency MESFET""s has turned into a commercial reality. Such higher frequency devices are extremely useful in a number of applications, some of the more familiar of which are power amplifiers, wireless transceivers such as cellular telephones, and similar devices.
In turn, because the wide bandgap characteristics of silicon carbide and the Group III nitrides theoretically permit devices to be designed for operation at higher frequencies and power levels, the high-resistivity substrates used for MESFETs and related devices must generally meet stricter criteria than high-resistivity or semi-insulating substrate materials used for similar devices in other materials with narrower band gaps; e.g., gallium arsenide (GaAs). Accordingly, as these higher frequency devices are developed and used, there exists an increasing demand for high quality semi-insulating silicon carbide substrates that will permit the electronic properties of wide-band gap epitaxial layers to be fully exploited.
Furthermore, the superior energy barrier produced by a high quality high-resistivity substrate offers the potential for increasing and maximizing the breakdown voltage (VB) of semiconductor devices.
In conventional semi-insulating silicon carbide substrates (and in some respects, xe2x80x9cconventionalxe2x80x9d represents a relatively recent time period), the appropriate resistivity has been obtained by doping the silicon carbide with vanadium. Vanadium creates a energy level that is about midway between the valence and conduction band in silicon carbide; i.e. at about 1.5 eV from either band edge in the 2.99 eV SiC bandgap. Doping silicon carbide with vanadium also tends to result in the vanadium compensating the residual donors (nearly always nitrogen) and acceptors (less frequently present) in silicon carbide to produce a material that is nearly insulating at room temperature. A description of such material is set forth, for example, in U.S. Pat. No. 5,611,955.
Although vanadium-doped silicon carbide is useful as a semi-insulating material for some purposes, it maintains some characteristic disadvantages. In particular, because of its amphoteric characteristics vanadium can produce either a donor or acceptor level in silicon carbide. More specifically, vanadium""s acceptor level is located relatively close to the conduction band in silicon carbide. As a result, thermal excitation of electrons can occur at moderately high temperatures, thus reducing the resistivity and correspondingly reducing the functional characteristics of devices made from these materials at such slightly elevated temperatures. See, e.g., Mitchel, et. al. xe2x80x9cFermi-Level Control and Deep Levels in Semi-insulating 4H-SIC.xe2x80x9d Journal of Applied Physics, Volume 86, No. 9, Nov. 1, 1999. Additionally, in operation, electrons can get trapped in vanadium""s extra level in a silicon carbide substrate, causing a time delay after injection. This tends to build up an internal charge in the semi-insulating layer, with a corresponding accumulation of holes in the epitaxial layer (speaking with reference to a MESFET) and this built-up charge does not follow the frequency being applied to the gate.
In order to avoid the problems introduced by vanadium that prevent silicon carbide and other wide-band gap devices from reaching their potential in high frequency applications, the assignee of the present invention has developed a semi-insulating silicon carbide substrate material that avoids the use of vanadium. A description and discussion of this subject matter is set forth in commonly assigned U.S. Pat. No. 6,218,680 for xe2x80x9cSemi-Insulating Silicon Carbide Without Vanadium Dominations,xe2x80x9d and its Continuation, Ser. No. 09/757,950 filed Jan. 10, 2001. The contents of these are incorporated entirely herein by reference.
Although the material described in the ""680 patent offers significant advantages over vanadium-doped material, production of the material requires dealing with the generally high native background amount of nitrogen that is almost always present in silicon carbide. The ""680 patent does so in terms of compensation techniques. Even though such techniques have been proven successful, the presence of nitrogen still tends to raise difficulties and inefficiencies in the manufacturing or growth process. In particular, during sublimation growth of silicon carbide (e.g., U.S. Pat. No. RE34,861), the concentration of nitrogen tends to vary over the course of time. Additionally, for reasons that tend to be empirically observed rather than conceptually explained, the concentration of nitrogen in a growing crystal of silicon carbide tends to vary along the geometry of the crystal. Thus, although the technique of the ""680 patent has proven to be a significant improvement, it requires the elimination of nitrogen, or at a minimum, its careful control.
Therefore, it is an object of the present invention to provide a high-resistivity silicon carbide substrate that has the characteristics required for successful high frequency devices, and that can be produced over a somewhat wider range of operating parameters, while still avoiding the disadvantages of vanadium and related dopants.
The invention meets this object with a high-resistivity silicon carbide single crystal that includes at least one compensated dopant having an electronic energy level far enough from an edge of the silicon carbide bandgap, or at low enough concentrations (or both), to avoid conductive behavior, while far enough from mid-gap towards the band edge to create a greater band offset than do mid-level states (e.g. traps, defects, or dopants) when the substrate is in contact with a doped silicon carbide epitaxial layer and when the net amount of the dopant present in said crystal is sufficient to pin the Fermi level at the dopant""s electronic energy level, with the silicon carbide crystal having a resistivity of at least 5000 ohms-centimeters at room temperature (298K).
In preferred embodiments, the high-resistivity crystal or substrate is used in conjunction with an adjoining epitaxial layer to produce a desired band-bending effect described in more detail later herein. Furthermore, it will be understood that although an epitaxial layer represents a presently-preferred embodiment, the desired offset can also result when the high resistivity portion is adjacent any appropriate active region, including regions formed by implantation or diffusion doping rather than as distinct epitaxial layers.
As used herein, the net amount of the dopant refers to the amount that acts in doping fashion; i.e., with or without compensation by other elements or by other items such as point defects.
In this regard, those of skill in this art recognize that dopants near the edge of the bandgap are more likely to produce conductive behavior than are dopants that produce levels near or at mid-gap. This is particularly so in wide bandgap materials such as silicon carbide.
In another aspect, the invention is a high-resistivity silicon carbide single crystal comprising nitrogen and at least one acceptor element having an electronic energy level of between 0.3 and 1.4 eV relative to (i.e., above) the valence band of monocrystalline silicon carbide, with the at least one acceptor element being present in an amount that overcompensates the nitrogen and pins the Fermi level of the silicon carbide substrate to the electronic energy level of the at least one acceptor element.
In another aspect the invention is a high-resistivity silicon carbide single crystal comprising an amount of electrically active nitrogen, an amount of electronically active point defects that act as acceptors, and an amount of at least one acceptor element having an electronic energy level of between 0.3 and 1.4 eV relative to the valence band of monocrystalline silicon carbide, with the combined amount of the acceptor element and the point defects being greater than the amount of electrically active nitrogen, and any other electrically active donors, including intrinsic point defects. The resulting compensation pins the Fermi level of the silicon carbide substrate to the electronic energy level of the at least one acceptor element.
In another aspect the invention is a high-resistivity bulk single crystal of silicon carbide comprising scandium, boron and non-intentionally introduced nitrogen (i.e., nitrogen typically present, but generally not from a proactive doping step), and with the concentration of nitrogen (and any other electrically-active donors, including intrinsic point defects) being greater than the concentration of scandium, and with the concentration of boron being sufficient for the sum concentration of boron and scandium to overcompensate the nitrogen (or other electrically active donor), and pin the Fermi level of the silicon carbide to the level of the scandium.
In yet another aspect, the invention is a high-resistivity silicon carbide single crystal comprising electrically active nitrogen and electrically active intrinsic point defects that act as donors, electrically active point defects that act as acceptors, a first acceptor element having an electronic energy level of between 0.3 and 1.4 eV relative to the valence band of monocrystalline silicon carbide, and a second acceptor element having an electronic energy level of between 0.3 and 1.4 eV relative to the valence band of monocrystalline silicon carbide, with the energy level of the first acceptor element being deeper than the energy level of the second acceptor element, and with the combined amount of the acceptor elements and the acceptor-acting point defects being greater than the amount of electrically active nitrogen (and donor acting point defects), thus pinning the Fermi level of the silicon carbide substrate to the electronic energy level of the first acceptor element.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings in which: